co-occurrence vs happening

co-occurrence and happening both are nouns.

Word	Noun	Adjective	Verb	Adverb
co-occurrence	Yes	No	No	No
happening	Yes	No	No	No

As nouns, happening is a hypernym of co-occurrence; that is, happening is a word with a broader meaning than co-occurrence:

co-occurrence: an event or situation that happens at the same time as or in connection with another
happening: an event that happens

Other hypernyms of co-occurrence include natural event, occurrence, occurrent.

co-occurrence (noun)	happening (noun)
the temporal property of two things happening at the same time	an event that happens
an event or situation that happens at the same time as or in connection with another